Various pathologies of the human spine may be treated by stabilizing the joints of the spine. A traditional approach has been to rigidly fix joints of the spine through a number of surgical techniques and approaches. More recently, therapies have been developed to restore proper spacing of the spinal anatomy without more drastically affecting the natural motion of the spine by rigidly fixing or fusing one or more spinal joints. These therapies are sometimes referred to as motion preserving or dynamic, or as joint replacements, when specific joints such as joints between vertebral bodies or facet joints are replaced or treated. However, there remains a significant need to effectively mimic the natural load bearing and dynamic characteristics of the complex structure of the human spine.
A significant challenge associated with implementing systems with flexible and/or articulating components is providing adequate fixation to the anatomy while simultaneously mimicking characteristics of the spine under various loading and motion conditions such as flexion, extension, and lateral bending. It may be particularly challenging to design a system that can support all loads developed over full ranges of flexion, extension, and lateral bending. The pedicle portions of vertebrae have traditionally provided excellent points to connect spinal rod and screw fixation systems because the pedicles include significant amounts of cortical bone into which a screw may be effectively placed. However, the posterior portion of the spine supported by fixation systems using pedicle screws is not the predominant natural load bearing structure of an anatomically correctly functioning spine. Therefore, a dynamic system that relies on pedicle screws may need a dynamic or flexible device connecting between the pedicle screws that is capable of responding non-linearly to loads applied in order to appropriately shares load with anterior portions of the spine. A need exists for improved pedicle-based implants that may be capable of both supporting a full range of anatomical loads developed over various degrees of flexion, extension, and lateral bending and sharing loads with anterior portions of connected vertebrae in a physiologically appropriate manner.